Spinal rods having different flexural rigidities about different axes and methods of use

ABSTRACT

A vertebral rod has an elongated body extending along a longitudinal axis. The rod also includes a cavity extending the length of the body. Either the body or the cavity may have an asymmetrical shape about a centroid in a plane perpendicular to the longitudinal axis. Alternatively, both may have the symmetrical shape about the centroid. The body of the rod may be bounded by an exterior surface and the cavity. The body has a first bending axis that is perpendicular to longitudinal axis. The body also has a second bending axis that is perpendicular to the longitudinal axis and to the first bending axis. The body of the rod may be distributed asymmetrically about the first and second bending axes. Also, the rod may have a different bending stiffness about the first and second bending axes.

BACKGROUND

Spinal or vertebral rods are often used in the surgical treatment ofspinal disorders such as degenerative disc disease, disc herniations,scoliosis or other curvature abnormalities, and fractures. Differenttypes of surgical treatments are used. In some cases, spinal fusion isindicated to inhibit relative motion between vertebral bodies. In othercases, dynamic implants are used to preserve motion between vertebralbodies. For either type of surgical treatment, spinal rods may beattached to the exterior of two or more vertebrae, whether it is at aposterior, anterior, or lateral side of the vertebrae. In otherembodiments, spinal rods are attached to the vertebrae without the useof dynamic implants or spinal fusion.

Spinal rods may provide a stable, rigid column that encourages bones tofuse after spinal-fusion surgery. Further, the rods may redirectstresses over a wider area away from a damaged or defective region.Also, a rigid rod may restore the spine to its proper alignment. In somecases, a flexible rod may be appropriate. Flexible rods may provide someadvantages over rigid rods, such as increasing loading on interbodyconstructs, decreasing stress transfer to adjacent vertebral elementswhile bone-graft healing takes place, and generally balancing strengthwith flexibility.

Aside from each of these characteristic features, a surgeon may wish tocontrol anatomic motion after surgery. That is, a surgeon may wish toinhibit or limit one type of spinal motion following surgery whileallowing a lesser or greater degree of motion in a second direction. Asan illustrative example, a surgeon may wish to inhibit or limit motionin the flexion and extension directions while allowing for a greaterdegree of lateral bending. However, conventional rods tend to besymmetric in nature and may not provide this degree of control.

SUMMARY

Illustrative embodiments disclosed herein are directed to a vertebralrod having an elongated body extending along a longitudinal axis. Therod also includes a cavity extending the length of the body. Either thebody or the cavity may have an asymmetrical shape about a centroid in aplane perpendicular to the longitudinal axis. Alternatively, both mayhave the symmetrical shape about the centroid. The body of the rod maybe bounded by an exterior surface and the cavity. The body has a firstbending axis that is perpendicular to longitudinal axis. The body alsohas a second bending axis that is perpendicular to the longitudinal axisand to the first bending axis. The body of the rod may be distributedasymmetrically about the first and second bending axes. Also, the rodmay have a different bending stiffness about the first and secondbending axes. The cavity may be contained within or intersect theexterior surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of first and second assemblies comprisingspinal rods attached to vertebral members according to one or moreembodiments;

FIG. 2 is a lateral view of a spinal rod according to one or moreembodiments; and

FIGS. 3-20 are axial views of a spinal rod illustrating cross sectionsaccording to different embodiments.

DETAILED DESCRIPTION

The various embodiments disclosed herein are directed to spinal rodsthat are characterized by a cross section that provides differentflexural rigidities in different directions. Various embodiments of aspinal rod may be implemented in a spinal rod assembly of the typeindicated generally by the numeral 20 in FIG. 1. FIG. 1 shows aperspective view of first and second spinal rod assemblies 20 in whichspinal rods 10 are attached to vertebral members V1 and V2. In theexample assembly 20 shown, the rods 10 are positioned at a posteriorside of the spine, on opposite sides of the spinous processes S. Spinalrods 10 may be attached to a spine at other locations, including lateraland anterior locations. Spinal rods 10 may also be attached at varioussections of the spine, including the base of the skull and to vertebraein the cervical, thoracic, lumbar, and sacral regions. In oneembodiment, a single rod 10 is attached to the spine. Thus, theillustration in FIG. 1 is provided merely as a representative example ofone application of a spinal rod 10.

In one embodiment as illustrated in FIG. 1, the spinal rods 10 aresecured to vertebral members V1, V2 by pedicle assemblies 12 comprisinga pedicle screw 14 and a retaining cap 16. The outer surface of spinalrod 10 is grasped, clamped, or otherwise secured between the pediclescrew 14 and retaining cap 16. Other mechanisms for securing spinal rods10 to vertebral members V1, V2 include hooks, cables, and other suchdevices. Examples of other types of retaining hardware include threadedcaps, screws, and pins. Spinal rods 10 are also attached to plates inother configurations. Thus, the exemplary assemblies 12 shown in FIG. 1are merely representative of one type of attachment mechanism.

The rod 10 may be constructed from a variety of surgical gradematerials. These include metals such as stainless steels, cobalt-chrome,titanium, and shape memory alloys. Non-metallic rods, including polymerrods made from materials such as PEEK and UHMWPE, are also contemplated.Further, the rod 10 may be straight, curved, or comprise one or morecurved portions along its length.

FIG. 2 shows a spinal rod 10 of the type used in the exemplary assembly20 in FIG. 1. The rod 10 has a length between a first end 17 and asecond end 18 extending along a longitudinal axis A. Other Figuresdescribed below show various embodiments of a spinal rod 10characterized by different cross sections viewed according to the viewlines illustrated in FIG. 2. For instance, FIG. 3 shows one examplecross section of the spinal rod 10 a. In this embodiment, the spinal rod10 a is comprised of an oval or elliptical outer surface 22 a and aninterior cavity or aperture 30 a defined by an inner surface 32 a. Inone embodiment, the outer surface 22 a and inner surface 32 a areuniformly consistent along the entire length L of the rod 10 a. That is,the cross section shown in FIG. 3 may be the same at all points alongthe length L of the rod 10 a. The same may also be true of other crosssections described below. In one or more embodiments, the cross sectionof a rod 10 may vary along the length L of the rod 10.

The structural characteristics of the rod 10 may be dependent uponseveral factors, including the material choice and the cross sectionshape of the rod 10. The flexural rigidity, which is a measure ofbending stiffness, is given by the equation:Flexural Rigidity=E×I  (1)where E is the modulus of elasticity or Young's Modulus for the rodmaterial and I is the moment of inertia of a rod cross section about thebending axis. The modulus of elasticity varies by material and reflectsthe relationship between stress and strain for that material. As anillustrative example, titanium alloys generally possess a modulus ofelasticity in the range between about 100-120 GPa. By way of comparison,implantable grade polyetheretherketone (PEEK) possesses a modulus ofelasticity in the range between about 3-4 Gpa, which, incidentally, isclose to that of cortical bone.

In general, an object's moment of inertia depends on its shape and thedistribution of mass within that shape. The greater the concentration ofmaterial away from the object's centroid C, the larger the moment ofinertia. In FIG. 3, the moments of inertia about the x-axis I_(x) andthe y-axis I_(y) for the area inside the elliptical outer shape 22 a(ignoring the inner aperture 30 a for now) may be determined accordingto the following equations:I _(x) =∫y ²dA  (2)I _(y) =∫x ²dA  (3)where y is the distance between a given portion of the elliptical areaand the x-axis and x is the distance between a given portion of theelliptical area and the y-axis. The intersection of the x-axis andy-axis is called the centroid C of rotation. The centroid C may be thecenter of mass for the shape assuming the material is uniform over thecross section. Since dimension h in FIG. 3 is larger than dimension b,it follows that the moment of inertia about the x-axis I_(x) is largerthan the moment of inertia about the y-axis I_(y). This means that theoval shape defined by the outer surface 22 a has a greater resistance tobending about the x-axis as compared to the y-axis.

The actual bending stiffness of the rod 10 a shown in FIG. 3 may alsodepend upon the moment of inertia of the inner aperture 30 a.Determining the overall flexural rigidity of the rod 10 a requires ananalysis of the composite shape of the rod 10 a. Generally, the momentof inertia of a composite area with respect to a particular axis is thesum (or difference in the case of a void) of the moments of inertia ofits parts with respect to that same axis. Thus, for the rod 10 a shownin FIG. 3, the overall flexural rigidity is given by the following:I _(x) =I _(xo) −I _(xi)  (4)I _(y) =I _(yo) −I _(yi)  (5)where I_(xo) and I_(xi) are the moments of inertia about the x-axis forthe outer and inner areas, respectively. Similarly, I_(yo) and I_(yi)are the moments of inertia about the y-axis for the outer and innerareas, respectively.

In the present embodiment of the rod 10 a shown in FIG. 3, the inneraperture 30 a is symmetric about the centroid C. Consequently, themoments of inertia about the x and y axes for the area inside the outersurface 22 a are reduced by the same amount according to equations (4)and (5). Still, the overall flexural rigidity of the rod 10 a is greaterabout the x-axis as compared to the y-axis. Accordingly, a surgeon mayelect to install the rod 10 a in a patient to correspondingly controlflexion, extension, or lateral bending. One may do so by orienting therod 10 a with the x-axis positioned perpendicular to the motion that isto be controlled. For example, a surgeon who elects to control flexionand extension may orient the rod 10 a with the stiffer bending axis(x-axis in FIG. 3) approximately parallel to the coronal plane of thepatient. Conversely, a surgeon who elects to control lateral bending mayorient the rod 10 a with the stiffer bending axis (x-axis in FIG. 3)approximately parallel to the sagittal plane of the patient. The surgeonmay also elect to install the rod 10 a with the x and y axes oriented atangles other than aligned with the sagittal and coronal planes of thepatient.

It may be desirable to adjust the bending stiffness of the rod 10 byvarying the size and shape of the inner aperture 30. For instance, asurgeon may elect to use the rods 10 disclosed herein with existingmounting hardware such as pedicle screws or hook saddles (not shown).Some exemplary rod sizes that are commercially available range betweenabout 4-7 mm. Thus, the overall size of the rods 10 may be limited bythis constraint.

FIG. 4 shows a rod 10 b similar to rod 10 a (i.e., outer surface 22 b issubstantially similar to surface 22 a) with the exception that the inneraperture 30 b defined by inner surface 32 b is larger than the inneraperture 30 a of rod 10 a. Using the equations above, one is able todetermine that the overall flexural rigidity about the x and y axes isgreater for rod 10 a as compared to rod 10 b. Rods 10 a and 10 b may beavailable as a set with a common outer surface 22 a, 22 b. However,since the rods have a different internal aperture 30 a, 30 bconfiguration, a surgeon may select between the rods 10 a, 10 b to matcha desired bending stiffness.

The internal aperture 30 may be asymmetric as well. For example, the rod10 c shown in FIG. 5 includes an outer surface 22 c that issubstantially similar to the outer surface 22 a of rod 10 a. However,the inner aperture 30 c defined by surface 32 c is elliptical or ovalshaped. The inner aperture 30 c has a height h₁ parallel to the x-axisthat is less than the width b₁ parallel to the y-axis. That is, themoment of inertia of the inner aperture 30 c is greater about the y-axisthan about the x-axis. This is in contrast to the outer surface 22 c,which has a larger moment of inertia about the x-axis.

The rods 10 may also have multiple inner apertures 30. For instance, therod 10 d shown in FIG. 6 comprises a plurality of apertures 30 d, 130 ddefined by inner surfaces 32 d, 132 d. The outer surface 22 d may besubstantially similar to the outer surface 22 a of rod 10 a. Notably,the exemplary apertures 30 d, 130 d are disposed within the interior ofthe rod 10 d. Further, the apertures 30 d, 130 d are offset from thecentroid C.

The embodiments described above have all had a substantially similar,oval shaped outer surface 22. Certainly, other shapes are possible asillustrated by the embodiment of the rod 10 e shown in FIG. 7. Thisparticular rod 10 e has a square outer surface 22 e that issubstantially symmetric relative to axes X and Y. However, the inneraperture 30 e defined by inner surface 32 e is asymmetric relative tothese same X and Y axes. Inner surface 32 e is substantially rectangularand defined by dimensions b and h. Specifically, dimension b (parallelto the Y-axis) is not equal to dimension h (parallel to the X-axis). Inthe embodiment shown, dimension b is larger than dimension h. Therefore,the aperture 30 e has a larger moment of inertia relative to the Y-axisas compared to the X-axis. Consequently, according to equations (4) and(5), the rod 10 e has a greater bending strength about the X-axis ascompared to the Y-axis.

The rod 10 f shown in FIG. 8 has rectilinear inner 32 f and outer 22 fsurfaces. However, in contrast to rod 10 e, the inner surface 32 f issubstantially square and outer surface 22 f is substantiallyrectangular. This configuration is analogous to rod 10 a shown in FIG. 3in that the inner aperture 30 f is symmetric about the X and Y axeswhile the outer surface 22 f is asymmetric about the X and Y axes. Therod 10 g shown in FIG. 9 has both an inner aperture 30 g and an outersurface 22 g that are asymmetric about the X and Y axes. The same istrue of the rod 10 c shown in FIG. 5. However, rod 10 g has an inneraperture 30 g and an area inside the outer surface 22 g that have largermoments of inertia about the same X-axis. This is due, in part, to thefact that the rectangular inner aperture 30 g and outer surface 22 g aresubstantially aligned.

The rod 10 may also have substantially triangular outer surfaces 22 asevidenced by the embodiments 10 h, 10 i, and 10 j. In FIG. 10, the outersurface 22 h is shown as an isosceles triangle that has a larger heighth (parallel to the X-axis) than base b (parallel to the Y-axis). Thismay tend to yield a rod 10 h having a greater moment of inertia aboutthe X-axis. By comparison, the rod 10 i shown in FIG. 11 comprises atriangular outer surface 22 i that is substantially equilateral. The rod10 j shown in FIG. 12 comprises a substantially triangular outer surface22 j that is substantially equilateral, albeit with non-linear sides.The inner apertures 30 h, 30 i, 30 j may be shaped as shown in FIGS.10-12 or as desired in accordance with the discussion provided above.

Other rods 10 may have polygonal shapes such as the embodimentsillustrated in FIGS. 13 and 14. The rod 10 k shown in FIG. 13 comprisesa hexagonal outer surface 22 k while rod 10 m in FIG. 14 comprises apentagonal outer surface 22 m. The rods 10 may have more sides ifdesired.

The embodiments described thus far have included an aperture 30 that issubstantially contained within the interior of the outer surface 22. Inother embodiments, the aperture 30 may intersect with the outer surface22. This can be seen in the exemplary embodiments shown in FIGS. 15 and16. In FIG. 15, the rod 10 n comprises two apertures 30 n, 130 n thatare defined by inner surfaces 32 n, 132 n. As indicated, the innersurfaces 32 n, 132 n intersect the outer surface 22 n resulting in openapertures 30 n, 130 n. The rod 10 n is shaped similar to an I-beam thathas a greater moment of inertia and bending stiffness about the X-axis.By way of comparison, the rod 10 p shown in FIG. 16 also has a singleopen aperture 30 p defined by an inner surface 32 p that intersects withthe outer surface 22 p.

The rods 10 may also have a substantially circular outer surface 22similar to many conventional rods, thus accommodating existing rodsecuring hardware (not shown). This is illustrated by the exemplary rods10 q, 10 r, and 10 s shown in FIGS. 17, 18, and 19. In each case, theouter surface 22 q-s of the rod 10 q-s is substantially circular and/orcharacterized by a substantially constant radius. As such, the moment ofinertia about axes X and Y is substantially the same for the areaswithin the outer surface 22 q-s. However, the moment of inertia aboutthe X and Y axes for the rod 10 q-s may be altered by including anasymmetric inner aperture 30 q-s.

In FIG. 17, the inner aperture 30 q defined by inner surface 32 q has alarger moment of inertia about the X-axis. Thus, the rod 10 q has alarger moment of inertia about the Y-axis (pursuant to equations (4) and(5)). In FIG. 18, the inner aperture 30 r defined by inner surface 32 ris also substantially circular. However, the inner aperture 30 r isoffset from centroid C. Further, the inner surface 32 r is tangent tothe Y-axis, but spaced away from the X-axis. Thus, the moment of inertiaof the inner aperture 30 r is larger with respect to the X-axis ascompared to the Y-axis. Consequently, the moment of inertia and bendingstiffness of the overall rod 10 r is larger about the Y-axis.

FIG. 19 shows another embodiment of a rod 10 s having an open inneraperture 30 s. In this embodiment, the inner surface 32 s has asubstantially constant radius and intersects the substantially circularouter surface 22 s. The inner aperture 30 s is offset from the centroidC, but aligned with the Y-axis in the orientation shown. Therefore, theinner aperture 30 s has a larger moment of inertia about the X-axis. Thebending stiffness of the overall rod 10 s is therefore greater about theY-axis.

FIG. 20 shows the same rod 10 q as illustrated in FIG. 17. In thisparticular view, the rod 10 q comprises a first set of markings 34 (the− sign in the embodiment shown) and a second set of markings 36 (the +sign in the embodiment shown). The markings 34, 36 may be stamped,engraved, or otherwise included on the rod as an indication of thebending stiffness in the direction of the marking. The markings 34, 36may be included on an end 17, 18 of the rod 10 q as shown or on theouter surface 22 q.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper”, and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc and are also not intended to belimiting. Like terms refer to like elements throughout the description.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

The present invention may be carried out in other specific ways thanthose herein set forth without departing from the scope and essentialcharacteristics of the invention. For example, embodiments describedabove have contemplated one or two inner apertures 30 to modify themoments of inertia about one axis relative to another. The rods 10 donot need to be limited to this number of apertures. The moment ofinertia equations provided herein allow one to calculate moments ofinertia for any number of apertures and flexural rigidity of the overallrod 10. The present embodiments are, therefore, to be considered in allrespects as illustrative and not restrictive, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein.

1. A vertebral rod comprising: a body extending along a first axis andhaving a length along the first axis between a first end and a secondend; a cavity extending the length of the body; at least one of the bodyand the cavity having an asymmetrical shape about a centroid in a planeperpendicular to the first axis.
 2. The vertebral rod of claim 1 whereinthe cavity is centered about the first axis.
 3. The vertebral rod ofclaim 1 wherein the body is centered about the first axis.
 4. Thevertebral rod of claim 1 wherein the body and the cavity are centeredabout the first axis.
 5. The vertebral rod of claim 1 wherein the cavityis defined by an inner surface having a first shape and the body isdefined by an outer surface having a second shape, first and secondshapes being different.
 6. The vertebral rod of claim 1 wherein thecavity is defined by an inner surface having a first shape and the bodyis defined by an outer surface having a second shape, first and secondshapes being the same.
 7. The vertebral rod of claim 1 furthercomprising markings indicating an orientation of the asymmetrical shape.8. A vertebral rod comprising: a body having a cavity, each extendingalong a first axis and each having a length along the first axis betweena first end and a second end, the body bounded by an exterior surfaceand the cavity; the body having a first bending axis that isperpendicular to the first axis and a second bending axis that isperpendicular to the first axis and to the first bending axis, the bodybeing distributed asymmetrically about the first and second bendingaxes.
 9. The vertebral rod of claim 8 wherein the exterior surface isasymmetric about the first and second bending axes.
 10. The vertebralrod of claim 8 wherein the cavity is asymmetric about the first andsecond bending axes.
 11. The vertebral rod of claim 8 wherein theexterior surface and the cavity are each asymmetric about the first andsecond bending axes.
 12. The vertebral rod of claim 8 wherein the cavityis interior to the exterior surface.
 13. The vertebral rod of claim 8wherein the cavity intersects with the exterior surface.
 14. Thevertebral rod of claim 8 further comprising markings indicating anorientation of the asymmetric distribution of the body.
 15. A vertebralrod comprising: a body extending along a first axis and having a lengthalong the first axis between a first end and a second end, the bodyhaving a first cross sectional shape substantially perpendicular to thefirst axis; a cavity extending the length of the body, the cavity havinga second cross sectional shape substantially perpendicular to the firstaxis; and the first cross sectional shape and the second cross sectionalshape being different.
 16. The vertebral rod of claim 15 wherein thesecond cross sectional shape is interior to the first cross sectionalshape.
 17. The vertebral rod of claim 15 wherein the second crosssectional shape intersects the first cross sectional shape.
 18. Avertebral rod comprising: a body having a cavity, each extending along afirst axis and each having a length along the first axis between a firstend and a second end, the body bounded by an exterior surface and thecavity; the body having a first bending axis that is perpendicular tothe first axis and a second bending axis that is perpendicular to thefirst axis and to the first bending axis, the body having different areamoments of inertia about the first and second bending axes.
 19. Thevertebral rod of claim 18 wherein the exterior surface defines an areahaving different area moments of inertia about the first and secondbending axes.
 20. The vertebral rod of claim 18 wherein the cavity hasdifferent area moments of inertia about the first and second bendingaxes.
 21. The vertebral rod of claim 18 wherein the exterior surface andthe cavity each have different area moments of inertia about the firstand second bending axes.
 22. The vertebral rod of claim 18 wherein thecavity is interior to the exterior surface.
 23. The vertebral rod ofclaim 18 wherein the cavity intersects with the exterior surface. 24.The vertebral rod of claim 18 further comprising markings indicating anorientation of the asymmetric area moments of inertia.